Grate or mince the onion.
Place all of the ingredients except the butter/oil in a large bowl.
Mix thoroughly, using a spoon or your hands, for three to five minutes until completely combined into a smooth batter. You can also use a stand mixer on medium speed for about one minute.
Cover the bowl containing the meat mixture with plastic wrap and place it in the refrigerator for about 20 minutes.
Heat butter and/or oil in a large skillet over medium heat. Use enough to cover the bottom of the skillet.
Drop large spoonfuls of the turkey mixture into the preheated skillet. Flatten slightly with the back of a spoon and let cook for 8-10 minutes, until dark brown on one side.
Flip the frikadeller using a spatula and cook for another 8-10 minutes on the other side, or until cooked through. Add butter as needed.
Scoop the frikadeller out of the skillet with a spatula and serve. You may drain them on a paper towel prior to serving if you prefer.